DIAGNOSIS Background graying or vertical streaks are usually a cleaning issue, not a toner issue. The most likely cause is a dirty corona wire inside the drum unit or, less commonly, a dirty fuser roller.
FIX Remove the drum unit (the DR310). On the side, you'll see a thin green slider or tab. Slide it back and forth its full range 3-4 times. This cleans the corona wire inside that helps transfer toner. Reinstall the drum and run a test print.
IF THAT DIDN'T WORK If the streak is a smeared, greasy-looking line, the fuser might be dirty. This isn't user-serviceable, but you can try printing 5-10 blank pages (send a blank document) which can sometimes clean it via the paper passing through. If it persists, the fuser unit needs replacement.
HEADS UP Using non-Brother toner, even if it's 'compatible,' is a huge culprit for background and spillage. The powder formulation and particle size can be off, leading to excess toner not being properly cleaned off the drum.